1. Field of the Invention
This invention relates to formulations for imparting desired physical properties to gypsum slurries and particularly for gypsum slurries used in the manufacture of gypsum wallboards.
2. Description of the Prior Art
The production of gypsum based materials first involves the preparation of a slurry of calcium sulfate hemihydrate with water. The mixture is allowed to hydrate, or set to the harder calcium sulfate dihydrate, and the excess water is removed by drying through conventional methods. Gypsum products such as wallboard panels comprise a hardened gypsum core sandwiched between paper liners, which form the outside surfaces of the wallboard. One of the problems associated with wallboard panels relates to water or moisture absorption by the hygroscopic gypsum core. Such moisture absorption leads to the weakening of the wall board. To address this moisture absorption problem, the prior art teaches the addition of wax or asphalt compounds to the gypsum slurry to impart hydrophobic qualities to the gypsum core of the board. However, the wax compounds are not readily dispersed within the aqueous gypsum slurry. To address this issue, the prior art, such as U.S. Pat. No. 5,437,722, teaches the inclusion of additives to act as emulsifiers in the slurry. One of the known additives is polyvinyl alcohol (PVA).
Polyvinyl alcohol is added to the gypsum slurry in order to increase the mechanical strength of the wallboard. The polyvinyl alcohol coats the gypsum crystals/particles such that, during the setting of the wallboard, the coated gypsum particles are prevented from returning to the hemihydrate state from the dihydrate. However, the same effects of PVA on the gypsum slurry also has the unwanted side effect of increasing the viscosity (i.e. decreasing the flowability) of the slurry.
As can be understood, the addition of polyvinyl alcohol causes a thickening of the gypsum slurry. In such case, the slurry becomes too thick and is unable to be easily spread. As will be understood, in the manufacture of wall boards or panels this decreased flowability of the gypsum slurry leads to various problems. Firstly, the difficulty in spreading the slurry across the surface of the paper face severely affects the production time required in manufacturing process. Further, the thickening of the slurry leads to the formation of blisters in the wall board during the manufacturing process as explained further below. A further unwanted defect in the wall board is the separation of the paper liner from the hardened gypsum core of the board. This arises from the inability of the slurry to spread and dry over a period of time sufficient to allow the formation of a bond between the slurry with the paper. A poor bond may cause the paper liner to lift up and away from the core leading to production delays and lost revenue.
Attempts have, been made to solve the above-mentioned by problems by either avoiding the use of PVA or adding water or dispersants to the gypsum slurry to increase the flow characteristics of the slurry. However, each of these alternatives is associated with certain problems. Firstly, the removal of the PVA component reduces the strength properties of the final product. The addition of increased amounts of water, although providing better flow properties, increases the drying time (such as within conventionally known dryers) required for the gypsum slurry forming the core of the wallboard to harden. This, of course, leads to increased production costs. It has also been found that the longer drying time leads to a product with reduced mechanical strength. This is due to the finding that the longer drying time hinders the formation of the stronger calcium sulphate dihydrate. A quicker drying time enables the formation of this stronger compound and, therefore, leads to a stronger wallboard.
As an alternative, various dispersants have been proposed for addition to the gypsum slurry containing PVA. Two of the most commonly used dispersants are lignosulphonate, often obtained as a by-product of the pulp and paper industry, and sulphonated naphthalene formaldehyde condensate. There are however, some problems associated with the addition of these dispersants. Firstly, naphthalene compounds generally exhibit a cross-linking reaction in the presence of polyvinyl alcohol, thus producing a thickening of the slurry and decreasing flowability. With a thicker slurry, there is a tendency for the formation of voids within the hardened product. The voids trap water from the slurry and, on heating (for drying the slurry), the water from within these voids is converted to steam. These steam pockets lead to blisters on the surface of the board. In addition, the voids discussed above may also trap air in the form of air bubbles. Such bubbles reduce the mechanical strength of the wallboard.
The addition of lignosulphonate while improving the flowability of the slurry mixture often results in an increase in the required time for the slurry to set. This increased time greatly increases the manufacturing costs.
The present invention seeks to overcome the above described deficiencies in the prior art